Slime molds eat bacteria, yeasts, fungal spores, and other microorganisms found in soil and decaying organic matter. They consume this food primarily through phagocytosis, a process in which a cell physically engulfs a food particle and digests it internally. What makes this seemingly simple feeding strategy remarkable is how sophisticated it actually is: slime molds can selectively choose among bacterial prey, balance their nutrient intake with precision, and even “farm” bacteria for future meals.
What Counts as Food for a Slime Mold
Slime molds are found on organic substrates such as rotting logs, leaf litter, and damp soil, where they feed on the microorganisms colonizing those surfaces.1PubMed Central. Substrate composition directs slime molds behavior Bacteria are the primary food source for most species. The well-studied social amoeba Dictyostelium discoideum, a cellular slime mold that lives in soil, feeds almost exclusively on bacteria.2PubMed Central. Eat Prey, Live: Dictyostelium discoideum As a Model for Cell-Autonomous Defenses The plasmodial slime mold Physarum polycephalum, the large yellow organism often seen in time-lapse videos crawling across forest floors, also feeds on bacteria and fungal propagules, along with yeasts and other microbial life embedded in decaying plant material.
In forest ecosystems, slime molds across their amoeboid and plasmodial stages ingest bacteria, yeasts, and fungal spores in quantities large enough to meaningfully regulate microbial populations in the soil and litter layer.3Global Ecology and Conservation. Functional ecological role of slime moulds (Eumycetozoa) in forest biodiversity and conservation – Section: Element cycling roles of slime moulds (Eumycetozoa) in forest detrital networks They are generalist predators in the microbial world, capable of eating a wide variety of prey, though not every bacterial species is equally appetizing or equally easy to handle.
How Phagocytosis Works
Slime molds eat by phagocytosis, the same basic mechanism your white blood cells use to destroy invading pathogens. The cell membrane extends around a food particle, wraps it up in a membrane-bound pocket called a phagosome, and pulls it inside the cell. Once inside, the phagosome merges with compartments containing digestive enzymes that break down the prey into usable nutrients. The leftover waste gets expelled.
Research on the cellular slime mold Polysphondylium pallidum has quantified this process in detail. When researchers fed these amoebae tiny polystyrene beads as a stand-in for bacteria, the cells ingested particles most efficiently when they were roughly one micrometer in diameter, approximately the size of many common bacteria.4PubMed Central. Phagocytosis by the cellular slime mold Polysphondylium pallidum during growth and development The phagocytic machinery appears tuned to particles in that size range. Interestingly, the same study found that phagocytosis slowed down when protein was added to the surrounding medium, when salt concentration increased, or when aerobic metabolism was blocked, suggesting that engulfing food is an energy-intensive activity that depends on the cell’s metabolic state.
For plasmodial slime molds like Physarum polycephalum, the process is fundamentally similar but happens on a larger scale. The plasmodium is a single enormous cell with many nuclei, spreading out like a network of veins. As the leading edges of this network encounter bacteria or other food particles, the membrane engulfs them in the same phagocytic fashion. Nutrients are then distributed throughout the organism by a rhythmic back-and-forth streaming of the cytoplasm, a pulsing flow visible to the naked eye.
How Slime Molds Find Their Food
Finding bacteria in a dark, complex environment like soil or leaf litter requires some method of detection, and slime molds rely heavily on chemical sensing. They move toward certain chemicals released by their prey or by decomposing organic matter. In Physarum polycephalum, researchers tested the organism’s response to a range of simple volatile organic chemicals and found clear preferences. The slime mold moved most strongly toward farnesene and beta-myrcene, both terpenes associated with plant material and microbial activity, while showing weaker attraction to other compounds.5PubMed Central. Assessing the chemotaxis behavior of Physarum polycephalum to a range of simple volatile organic chemicals – Section: Results and Discussion This chemical navigation allows the organism to grow preferentially toward zones where food is likely to be abundant.
Even more striking, the amoeboid cells of Dictyostelium discoideum can discriminate among different bacterial species before they even start actively feeding. Research has shown that dormant spores of D. discoideum respond differently to different bacteria, hatching at higher rates in the presence of nutritionally valuable species. When given a choice, the amoebae formed larger feeding zones on high-quality prey and actively preferred certain bacteria over others.6Functional Ecology. A dormant amoeba species can selectively sense and predate on different soil bacteria This is not random grazing. The organism evaluates its options from the moment it wakes up.
Not All Bacteria Are Equal
While slime molds are considered generalist predators capable of consuming many different bacterial species, this generalism comes with real tradeoffs. When D. discoideum feeds on multiple prey species that naturally co-occur in soil, it appears to use partially different cellular methods for different prey. Switching between prey types, or handling several types simultaneously, imposes a cost on the organism’s growth and efficiency.7PubMed Central. Costs of being a diet generalist for the protist predator Dictyostelium discoideum Being able to eat almost anything is an advantage in an unpredictable environment, but it is not free.
From the bacteria’s perspective, being eaten is not inevitable. Many soil bacteria have evolved chemical defenses against predators like slime molds. Some secrete small toxic molecules that can kill or repel the attacking amoeba.8Synlett. The Role of Bacterial Natural Products in Predator Defense This creates an ongoing evolutionary arms race in the soil: slime molds evolve better ways to detect and consume prey, while bacteria evolve better ways to avoid being consumed. Some bacterial species are consistently avoided by slime molds, likely because the energy cost of overcoming their defenses outweighs the nutritional benefit.
Balancing the Diet
One of the more surprising discoveries about slime mold feeding behavior is how precisely they can balance their nutrient intake. Physarum polycephalum does not simply eat whatever it encounters; it adjusts how much it takes from different food sources based on their nutritional content. When researchers placed patches of food with different protein-to-carbohydrate ratios around a growing slime mold, the organism extended its network to contact each patch in the precise proportions needed to compose an optimal diet.9PubMed Central. Amoeboid organism solves complex nutritional challenges It allocated more growth toward the nutrient source it needed more of, and less toward the one it already had enough of.
Follow-up work confirmed that P. polycephalum can choose diets based on protein-to-carbohydrate content to support its optimal growth rate.10PubMed Central. Diet and mitonuclear haplotype interactions affect growth rate in a slime mould This is a sophisticated behavior for an organism with no brain, no nervous system, and no obvious decision-making apparatus. It has fueled a lot of interest in understanding how single cells or simple networks can perform tasks that look, from the outside, like intelligent choices.
What Happens When Food Runs Out
Slime molds have dramatic responses to starvation, and these responses differ between species. In Physarum polycephalum, the plasmodium contracts and consolidates when food becomes scarce. At the cellular level, glucose deprivation triggers a significant increase in the number of mitochondria inside the cell. Researchers found that when glucose was absent, the slime mold appeared to ramp up mitochondrial production, likely because it was switching to metabolic pathways that run inside mitochondria to extract energy from alternative fuel sources.11PubMed Central. Mitochondrial numbers increase during glucose deprivation in the slime mold Physarum polycephalum The organism does not just passively starve; it actively retools its metabolic machinery.
In cellular slime molds like D. discoideum, starvation triggers a fundamentally different survival strategy. Individual amoebae, which normally live and feed independently, begin aggregating into a multicellular structure. This collective eventually forms a fruiting body made of a stalk of dead cells supporting a cluster of reproductive spores at the tip.12PubMed Central. Fitness tradeoffs between spores and nonaggregating cells can explain the coexistence of diverse genotypes in cellular slime molds The spores can survive harsh conditions and, when they land in a favorable spot, hatch into new amoebae that resume feeding. Starvation, in other words, is the trigger for the entire multicellular phase of the life cycle.
Bacteria Farmers
Perhaps the most remarkable feeding-related behavior discovered in slime molds is a form of primitive agriculture. About a third of wild-collected clones of D. discoideum engage in what researchers describe as farming. Instead of consuming every bacterium in their patch before forming fruiting bodies, these “farmer” clones stop eating early. They incorporate live bacteria into their fruiting bodies and carry them along inside their spores as the spores disperse to new locations.13PubMed. Primitive agriculture in a social amoeba When the spores land and hatch, the bacteria are released and can colonize the new environment, seeding a fresh food supply. This is a major advantage if the new site lacks edible bacteria.
This farming symbiosis is not purely beneficial. Farmer clones stably associate with certain bacterial partners and can pay a reproductive cost for maintaining the relationship. But they also gain capabilities that non-farmers lack, including the ability to carry food with them and, in some cases, defense against competitors.14PubMed Central. Burkholderia bacteria infectiously induce the proto-farming symbiosis of Dictyostelium amoebae and food bacteria Intriguingly, the farming trait can be induced by infection with certain Burkholderia bacteria, meaning the bacterial partner itself may drive the establishment of the symbiosis. The line between farmer and farmed gets blurry.
The Ecological Payoff of All That Eating
Slime mold feeding has consequences that ripple through the ecosystem. By consuming enormous quantities of bacteria, yeasts, and fungal spores, slime molds act as regulators of microbial populations in forest soils. When they digest their prey and excrete the waste, they release carbon, nitrogen, phosphorus, and sulfur that had been locked inside microbial cells back into forms that other organisms and plants can use.3Global Ecology and Conservation. Functional ecological role of slime moulds (Eumycetozoa) in forest biodiversity and conservation – Section: Element cycling roles of slime moulds (Eumycetozoa) in forest detrital networks This grazing-and-excreting cycle effectively speeds up nutrient recycling. Microcosm experiments suggest that sustained slime mold grazing can actually stimulate bacterial metabolism, increasing the rate at which carbon is broken down from litter and soil organic matter while promoting the release of nitrogen and phosphorus from microbial biomass.
This means slime molds are not just consumers; they are facilitators of decomposition. By keeping microbial populations in check and recycling the nutrients those microbes had absorbed, slime molds help maintain the fertility and functioning of forest soils. It is a role that rarely gets attention outside of soil ecology, but it has real implications for nutrient cycling and carbon sequestration at landscape scales.
Feeding in the Lab Versus the Wild
Most of what we know about slime mold feeding comes from laboratory studies, where conditions are controlled and food sources are standardized. In labs, Physarum polycephalum is commonly fed oat flakes, which support bacterial and yeast growth that the slime mold then consumes. Dictyostelium species are typically grown on bacterial lawns, often using Klebsiella or Escherichia coli as prey. These simplified setups have been invaluable for understanding the mechanics of phagocytosis, the kinetics of feeding, and the decision-making behavior of these organisms.
But wild conditions are far messier. In soil, a slime mold encounters dozens of bacterial species at once, along with fungal competitors, nematode predators, and constantly shifting moisture and temperature conditions. The costs of generalism documented in lab studies, where handling multiple prey types simultaneously reduces efficiency, are probably even more pronounced in these complex natural communities.7PubMed Central. Costs of being a diet generalist for the protist predator Dictyostelium discoideum At the same time, the chemical defense strategies of wild bacteria are likely more diverse and potent than what lab-cultured strains produce. Wild slime molds face a much tougher dining environment than their laboratory counterparts.
Why a Brainless Organism Looks So Smart
Much of the popular fascination with slime molds centers on their apparent intelligence, and feeding behavior is where this shows most clearly. A Physarum plasmodium can solve a nutritional optimization problem that a human dietitian would need a spreadsheet for, growing toward food patches in exactly the right proportions to balance its protein-to-carbohydrate intake.9PubMed Central. Amoeboid organism solves complex nutritional challenges A Dictyostelium spore can assess the nutritional value of nearby bacteria before it even hatches.6Functional Ecology. A dormant amoeba species can selectively sense and predate on different soil bacteria A farmer clone can exercise the restraint to stop eating before all the food is gone, preserving a seed stock for the future.
None of this requires a brain or even a neuron. These behaviors emerge from chemical signaling, membrane receptors, and the physical properties of cytoplasmic flow. The slime mold does not “decide” to eat one bacterium over another in the way you decide what to order for lunch. But the outcome, selective, efficient, sometimes even strategic foraging, looks strikingly like decision-making. This is part of why slime molds have become such popular research organisms: they force us to reconsider what intelligence really means, and whether a single cell can be smart in any meaningful sense. The feeding behavior, in all its specificity and sophistication, sits at the center of that question.